1. Field of the Invention
The present invention relates to a voice band signal cell assembly apparatus for use in ATM (Asynchronous Transfer Mode). The apparatus is capable of attaining a high transmission efficiency by reducing the number of cells formed in the voice CLAD (Cell Assembly and Diassembly) for ATM.
2. Description of the Prior Art
FIG. 1 is a block diagram illustrating a structure of a conventional cell assembly apparatus used for transmitting voice band signals in ATM. A voice band signal is divided into "voice" and "data". It should be noted that "data" refers to a signal produced by modulating a carrier so as to transmit data, such as digital data, by using the voice band.
FIG. 1 includes a voice detector 1, a high efficiency voice encoder 2, and a cell assembly section 3, which form a structure for assembling voice into cells. Such a structure has been disclosed in Japanese Patent Laid-Open Publication Nos. 4-157843 and 4-249446. If the voice band input signal is voice, then the high efficiency voice encoder 2 compresses the band of the voice signal and supplies the compressed voice signal to the cell assembly section 3. Assuming that a 64 kbps PCM signal is encoded into a 16K LD-CELP (ITU-T Rec. G. 728) signal with this compression efficiency, the signal band is then compressed to 1/4 of the original signal.
The voice signal is also input to the voice detector 1, where the presence or absence of voice in the voice signal is detected. In accordance with the detection result, the cell assembly section 3 forms the encoded voice signal into a cell, and sends the cell out onto an ATM transmission channel only when the voice detector 1 detects the presence of voice. Since about 40% of the voice signal has voice information (with 60% constituting periods of silence), a compression efficiency of about 1/2 is achieved by detecting the absence of voice. The total voice compression is about 1/8.
In the meantime, a cell assembly section 4 assembles data into cells. That is, if the voice band input signal is data, the data is input directly to the cell assembly section 4 and formed into cells. The cells are then sent out onto the ATM transmission channel.
Note that it is a discriminating section (I) 5 of FIG. 2 that determines whether the voice band input signal is voice or data. The discriminating section (I) 5 can be implemented by a technology such as the one recommended in ITU-T Rec. G. 763. Also note that a discriminating section (II) 6 of FIG. 2 is implemented by simply adding to the discriminating section (I) 5 a function to determine whether the data is a specific modulated wave or a modulated wave other than the specific modulated wave. The data of the specific modulated wave refers to data such as a G3 facsimile signal including a tone signal.
An advantage of such a conventional cell assembly apparatus for ATM is a high transmission efficiency achieved by way of high efficiency voice compression and compression of the silence parts in the signal, when the voice band input signal is voice. However, when the voice band input signal is supplied in the form of data, such as a G3 facsimile signal, the voice band input signal is input directly to the cell assembly section, and the voice band signal is not compressed. It is thus necessary to provide a wide band transmission channel for transmitting the voice band signal in the form of a cell. This is inconvenient because overall transmission efficiency is deteriorated.